The present invention relates to a method of connecting, through a universal joint, main pipes which are fixed to a member such as a riser pipe.
Hitherto, node-equipped riser pipes are used to connect a floating facility and a sea bottom facility for extracting crude oil buried under the sea bottom. In this riser pipe, adjacent riser pipe pieces are connected through a universal joint which permits these riser pipe pieces to be flexed in relation to each other.
On the other hand, the main pipe pieces fixed to the riser pipe pieces are connected through swivel joints to a relay pipe incorporated in the universal joint, such that an inclination of the upper and lower main pipe pieces in relation to each other is compensated for by the rotation of the swivel joint.
In addition, the main pipe piece itself is bent in the form of a loop to absorb the eccentricity or offset between the axis of a cross pin and the axis of the swivel joint which is increased as the wear of the bearing of the cross pin is increased.
In this conventional pipe connecting structure, however, it is necessary that the swivel joint be disposed such that the axis of the swivel joint coincides with the axis of the cross pin. Namely, there is a severe demand for the coincidence of axes between the swivel joint and the cross pin. Therefore, when the number of pipes is large, it is extremely difficult to connect the adjacent main pipe pieces through the universal joint and, in addition, the amount of radial projection of the expandable portion of the main pipe piece, extending in the radial direction of the riser pipe, becomes impractically large. At the same time, the path of the piping becomes extremely complicated.
Furthermore, this method which, for the compensation of the offset between axes of the cross pin and swivel joint, relies solely on expansion and contraction of the expandable portion of the main pipe piece, will not only increase the force to be imposed on the swivel joint but also shorten its life when the wear of the cross pin bearing becomes excessively large.